This invention pertains to apparatus for extending or lengthening bones and is particularly applicable to lengthening of human bones. There are many instances where a human being has one limb, particularly a leg, which is shorter than the other which could be congenital or due to an accident. It has been found possible to lengthen the deformed or shortened limb by devices which extend a proximal portion of a bone which has been severed from a distal portion. Such devices move the distal portion of the bone away from the proximal portion in small increments. This involves extending the gap between the severed bone portions in increments of approximately 1/4 mm at a time. When the bones are thus separated, new bone tissue grows in the extended gap as well as soft tissue surrounding the bone. It is possible to lengthen a bone such as a femur 10 or more centimeters over a period of a few months.
One such device was developed in the USSR by Gavriil A. Ilizarov et al. which essentially is an extendable cage which fits externally around a limb. The distal end of the cage is incrementally extended from the proximal portion mechanically. One of the problems with this type of device is that a plurality of pins are inserted through the muscle of the limb, and into both portions of the bone. With the proximal portion of the limb "anchored" by the pins in the cage, force is transmitted through the pins to pull the distal portion away from the anchored portion. Such a device is described in U.S. Pat. No. 4,615,338. One of the problems created by this type of device is that numerous incisions are created in a limb by the pins which are susceptible to infection and that the pins are continually pulling on the flesh. Another disadvantage is that the wearing of a cage over a period of months severely limits the mobility of the patient.
Numerous other devices employ an elongated rod or nail known as an intramedullary rod or nail. Such devices are inserted into the marrow cavity or medullary passage of the bone. These rods are often threaded and are anchored to the bone from within the medullar canal and are incrementally moved by nuts or rachet means to incrementally continue separating bone portions. One such early device is shown in U.S. Pat. No. 3,986,504 to Avila. The racheting or extending of the threaded rod is accomplished externally by racheting mechanism or wrenches. Again, this type of device lends itself to infection.
Other types of devices have been developed also in the USSR by Alexander Bliskunov and employ racheting devices which are implanted internally and are operated by the patient himself who moves portions of his body to activate the racheting mechanism. One problem with this type of device is that accidental movement or excessive movement can cause too much bone separation, disturbing the growth pattern. It is to these problems that the present invention is directed. Another objective is to provide a completely implantable, hydraulically operated mechanism with as little opportunity for infection as possible and which is substantially foolproof in operation.